1. Field of the Invention
The present invention relates to spread spectrum (SS) communication systems using Pseudo-Noise (PN) coding techniques and, more particularly, to priority communication management in a hybrid TDMA-SS system.
2. Prior Art
Spread spectrum (SS) systems, which may be Code Division Multiple Access (CDMA) systems, are well known in the art. SS Systems can employ a transmission technique in which a pseudo-noise (PN) PN-code is used as a modulating waveform to spread the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is de-spread using a synchronized replica of the PN-code.
In general, there are two basic types of SS systems: direct sequence spread spectrum systems (DSSS) and frequency hop spread spectrum systems (FHSS).
The DSSS systems spread the signal over a bandwidth fRF±Rc, where fRF represents the carrier frequency and Rc represents the PN-code chip rate, which in turn may be an integer multiple of the symbol rate Rs. Multiple access systems employ DSSS techniques when transmitting multiple channels over the same frequency bandwidth to multiple receivers, each receiver sharing a common PN code or having its own designated PN-code. Although each receiver receives the entire frequency bandwidth, only the signal with the receiver's matching PN-code will appear intelligible; the rest appears as noise that is easily filtered. These systems are well known in the art and will not be discussed further.
FHSS systems employ a PN-code sequence generated at the modulator that is used in conjunction with an m-ary frequency shift keying (FSK) modulation to shift the carrier frequency fRF at a hopping rate Rh. A FHSS system divides the available bandwidth into N channels and hops between these channels according to the PN-code sequence. At each frequency hop time a PN generator feeds a frequency synthesizer a sequence of n chips that dictates one of 2n frequency positions. The receiver follows the same frequency hop pattern. FHSS systems are also well known in the art and need not be discussed further.
In general, although the original data stream is recovered, after PN acquisition, the actual data cannot be recovered, or extracted from the data stream until data-symbol boundaries are identified. Data-symbol boundaries are identified either with a symbol synchronizer (bit synchronizer, with its attendant acquisition and pull-in time), or with PN code epochs.
Time division multiple access (TDMA) is a communications system that divides a single frequency channel into short-duration time slots to enable multiple users to transmit on the same channel. Hybrid TD-Spread Spectrum (SS) or TDMA-SS transmission systems employ a succession of short-duration PN encoded data bursts emanating from one or more communication stations.
A TDMA structure is composed of a stream of frames with a number of fixed-time slots per frame. Each time slot may be of an assigned type: entry and registration, routine maintenance, priority messages, mass data transfer, et cetera. The composition of slot types in a frame may be reassigned from frame to frame. A time slot in a frame may be assigned to one specific user; or a time slot, such as an entry-type time slot, may be a free-for-all slot; where any number of users may attempt to use it on a first-come, first-server basis.
For example, one station designated as the HUB may assign a certain number of satellite stations, designated as SPOKES, certain time slots with in a given window within which to communicate with the HUB. Yet, there are times when a SPOKE may need to communicate with a receiver, in other words, a priority communication, outside of its assigned time slot. In some applications this is accomplished by reserving an unassigned time slot for priority interrupts, such as shown in U.S. patent application Ser. No. 2002/0167959. Thus, SPOKEs may transmit priority interrupts which may be received and acted upon by the HUB. However, since any SPOKE, if there are more than one, may transmit during the unassigned slot time there is a possibility of collision and lost data.
Certain classes of TDMA users require minimal time latency before their priority communication is received. However, since a Priority Time Slot occupies a full time slot, it can be seen that data/message latency increases if Priority Time Slots are assigned to specific users; a user is blocked from using a Priority Time Slot until that specific user's Priority Time Slot occurs. In another words, a user may have to wait until another available priority interrupt time slot is available to transmit its priority message.
It is therefore desirable to provide a TDMA system where priority interrupt time slots are managed to prevent collisions, and therefore subsequent loss of data. The purpose is to reduce TDMA priority data/message latency and eliminate collisions (of priority users that without assigned priority time slots would compete and possibly collide while attempting to use the same priority time slot). It is also desirable to provide a TDMA system where TDMA priority data/message time latency is reduced and collisions are eliminated.